Academic Skepticism (idea)

There are some truths that are completely unknowable to people.
This is the credo of the Academic Skeptic. This branch of skepticism
comes from the Platonic Academy which gradually adopted
this position after Plato's death.

Do you know that knowledge is impossible? Either you cannot
know that, or some knowledge is possible.

This retort and its variations are the single most common objection
to skepticism. However, it applies to very few of the Academic
Skeptics, and to none of the other branches of skepticism. Still,
many philosophers believe that skepticism is self-refuting, as
Descartes said, that doubting cannot doubt itself.

Academic skeptics have been aware of this retort for centuries and are
certainly not foolish enough to engage it without proper defense.
Some followed Cicero and held that no knowledge is possible except
for the one truth that no other knowledge is possible. This exception
is hard to justify to some, but at least seems to free it of contradiction.
Others believe that quite a bit of knowledge is attainable, but the
unknowable is also very large and includes all the truly important
questions, such as the nature of humanity, the soul, the gods,
change, reality, free will, immortality, and the whys of ethics.

Protagoras declares that one can take either side on any question
and debate it with equal success - even on this very question, whether
every subject can be debated from either point of view. --Seneca

According to Cicero, at least four ancient philosophers took the
position that nothing can be known except the proposition that nothing
(else) could be known: Socrates, Varro, Antipater, and Cicero
himself. Others took the position that nothing could be known
including the proposition that nothing could be known: Arcesilas,
Carneades, and Chian Metrodorus.

Academic skeptics limited themselves to denying the possibility,
not of knowledgeper se, but of certainty. We might
chance upon the true explanation of something important, but we can
never know it because it would be uncertain and in now way discernible
from the competing false explanations.